Cannot Specify Length Expression

I'm creating a BPM method and need to evaluate if the JobHead.UserChar1 field does not have a value. I've tried using "the JobHead.UserChar1" field of the changed row is equal to the ? expression" to evaluate for Null value, but looking at the table the field is not null by default, thus this will not work.

Now I'm trying to use the expression "String(ttJobHead.UserChar1)=0" instead of the null value (?) in this same condition, but when I validate the syntax in the expression it gives the error:
** Incompatible data types in expression or assignment. (223)
** Could not understand line 1. (196)

How can I fix this or how else can I evaluate the UserChar1 field for no value?

All fields can be null. However, it should never be confused
with interger/decimal that is 0 or a character that is blank ("").
Also never confuse the ? value with "?". One is unknown, the
other is a single length character string with the value of
a question mark.

The null is a very special value treated entirely differently.
It fails *both* greater than or less than values. The only
test it succeeds at is = ?